1. Field of the Invention
The present invention relates to systems and methods for exhausting enclosed chambers and, more particularly, to a system and method for exhausting vapor from a heated pressure chamber.
2. Description of the Invention Background
Sterile solutions or fluids are useful for a variety of purposes in clinical practice. For example, solutions may be administered to a patient parenterally, given orally, applied to the skin or used to irrigate or bathe body tissues, organs and wounds. Such solutions must be sterilized prior to use. They are often sterilized in capped containers in which the solution can be stored for long periods while maintaining sterility.
Moist heat in the form of saturated steam under pressure is one of the most dependable agents for the destruction of all forms of microbial life. Steam sterilization is frequently used to sterilize liquids in containers. A problem incident to the steam sterilization of liquid loads is liquid loss due to boiling of the liquid during the exhaust phase of the sterilization cycle if the exhaust is too fast. Another problem is the extended cycle time caused by the current method of exhausting and cooling sterilization chambers.
Saturated steam is water vapor in the condition in which it is generated from water. During the heating process, steam formed near the surface of the heating element rises to the surface of the water and breaks through. The steam released into the space above the liquid is saturated steam. The steam-free water is known as saturated water and is at the same temperature as the saturated steam. The temperature is known as the saturation temperature. Saturated steam undergoes a reduction in temperature when it undergoes a reduction in pressure and vice versa. The relationship between temperature and pressure for saturated steam follows a curve shown in FIG. 1. The phase-boundary curve for saturated steam is also representative of the vapor pressure of water.
During the sterilization phase of a steam sterilization cycle, there is no liquid loss in the containers of liquid because the steam pressure in the chamber is equal to or above that which is necessary to induce boiling of the liquid in the containers. When the pressure is reduced in the exhaust phase following the sterilization phase, boiling of the liquid in the containers may occur if the pressure in the chamber is reduced faster than the temperature drop so that the pressure reaches a point lower than the pressure necessary to maintain the temperature of the liquid at its saturated steam point. If the exhaust rate is too rapid, violent boiling may occur, resulting in the explosion of the containers. The precise relationship between the liquid in the containers and the pressure in the chamber depends on the kind of liquid.
To avoid such dire consequences, some sterilization cycles employ exhaust rates based on the worst case load conditions. The worst case load assumption results in increased cycle times which in turn results in a considerable reduction in sterilizer efficiency. Exhaustion is generally non-linear and through a single fixed orifice. An alternative method heretofore used to avoid boiling of the liquid in the containers provides an exhaust rate which is adjustable by the operator to account for variations in load types and sizes. The adjustable exhaust rates which can be set in some commercially available sterilizers are linear. The operator adjusted exhaust rates increase the efficiency of the sterilization cycle but also increase the potential for human error. If the operator sets the exhaust rate too fast, boiling may occur and the containers may explode. To avoid the possibility of error, some operators set the exhaust rate lower than necessary, thereby decreasing efficiency and nullifying the advantage to an operator adjustable exhaust rate.
Thus, there is a need for a method for determining the optimal rate for exhausting vapor, such as steam, from a pressure chamber in a safe, efficient manner. There is a further need for such a method which is not subject to human error. Finally, there is a need for a method for determining the optimal rate for exhausting vapor from a pressure chamber which can be run automatically.